Freeboard and Load Lines

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Freeboard and Load Lines

Freeboard and draught describe different distances on a ship. The draught is the vertical distance between the current waterline and the bottom of the ship; assigned freeboard is measured amidships, vertically from the upper edge of the deck line to the upper edge of the corresponding load line. When cargo is loaded, the distance between the current waterline and the relevant load line changes; the assigned freeboard itself remains the specified distance between the deck line and the load line. The waterline changes with loading and external conditions.

Freeboard and the freeboard deck

The freeboard deck is the deck used as the basis for the calculation. Freeboard is the vertical distance from the deck line to the upper edge of the corresponding load line. It therefore does not describe how far the hull is currently submerged. A greater distance between the load line and the deck line means more of the hull lies above that mark; the current draught cannot be determined from this alone.

The distinction is important in practice: the assigned load line determines the permissible loading, while the actual draught indicates how much of the hull is underwater. A mark on the hull is no substitute for measuring the water depth in a port. Water depth describes the available space between the water’s surface and the bed of the body of water; it is a separate measurement.

Load lines on the hull

Under the International Convention on Load Lines, assigned load lines must be marked amidships on both sides of the ship. The deck line is positioned beside them to establish the reference to the freeboard deck. The markings must therefore be looked for on both sides of the hull; they are not general indicators of the depth of a ship or a harbor basin.

The height to which a ship may be loaded depends on more than the mark as a graphic symbol. The regulations take different areas of operation and seasonal conditions into account. As a result, different assigned lines may apply to the same ship depending on the zone and time of year. The relevant markings on the ship must be read together with the requirements applicable to the voyage. A single line, without its letter or zone reference, does not provide a complete indication of the current loading limit.

Why freeboard is required

Limiting a ship’s draught through a prescribed freeboard contributes to safety. The International Maritime Organization (IMO) states that the purpose of the rules, in addition to specifying freeboard, is to ensure the ship’s weathertightness and watertightness. The freeboard principle provides reserve buoyancy; the requirements also take adequate stability into account and are intended to prevent excessive hull stress caused by overloading. Technical regulations also cover openings such as doors, freeing ports and hatches, to ensure that the hull remains watertight below the freeboard deck.

The load lines indicate the permitted limit under the applicable conditions. How that limit applies to a particular ship depends on its assigned markings, the zone of operation and the season.

Freeboard, the load line and draught are related, but refer to different things: freeboard is a specified distance on the hull, the mark indicates the permissible loading limit, and draught is the portion extending below the water. For information about harbor basins and the distance to the bed of a body of water, water depth is the relevant measurement. Moulded depth is one of the dimensions of a ship’s hull.